As of Oracle9i, tablespaces are created as locally managed by default, so the extent
management local clause is optional when you create a new tablespace.
NOTE
If you make the SYSTEM tablespace locally managed, you can
only create locally managed tablespaces within the database; any
dictionary-managed tablespaces imported using the transportable
tablespace feature can only be opened read-only.
Identifying Chained Rows
When a data segment is created, a pctfree value is specified. The pctfree parameter tells the
database how much space should be kept free in each data block. The free space is used when
rows that are already stored in the data block extend in length via update operations.
If an update to a row causes that row to no longer completely fit in a single data block, that
row may be moved to another data block, or the row may be chained to another block. If you
are storing rows whose length is greater than the Oracle block size, you will automatically have
chaining.
Chapter 8: Database Tuning 259
Chaining affects performance because it requires Oracle to look in multiple physical locations
for data from the same logical row. By eliminating unnecessary chaining, you reduce the number
of physical reads needed to return data from a datafile.
You can avoid chaining by setting the proper value for pctfree during the creation of data
segments. The default value, 10, should be increased if your application will frequently update
NULL values to non-NULL values, or if long text values are frequently updated.
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